The past few years have seen some serious discussion among conservation biologists about the biodiversity value of invaded communities.  For example, to what degree do some invasive species integrate into native communities, how extensive and permanent are the damages from invasives, and are native species able to evolve in response?  It is important to resolve these questions regarding the ecology of invasives, in order to inform policy and management questions, such as: is it worth struggling to keep native communities pristine against the rising tide of invasions?  What is the implication of decelerating rates of invasion over time due to adaptation by natives, or switch by some native herbivores to browsing on the invasive?

An interesting question among this discussion about invaded communities concerns the puzzling observation that some rare plants still persist in areas heavily invaded by a non-native plant.  While invasive plants can and do drastically alter the physical landscape (e.g. honeysuckle, kudzu), they rarely lead (on their own) to complete extinction of native plants.  Why might this be?

A new study in the journal Science sheds some light on why rare plants are able to persist.  By studying highly invaded and relatively uninvaded plots of different sizes in three very different communities (Hawaii, Florida, and Missouri), Kristin Powell and colleagues found that invasive species do eliminate many species in local areas (several meters) but that across much broader areas (hundreds of meters), species abundance is hardly affected.  Invasive species clearly have strong effects, there is no doubt about that, but over broad scales native species find a refuge somehow.  One reason that native rare plants may be less affected by invasive plants than more common native plants is that rare plants may have a particular microhabitat to exploit, or they may perform well in the heavily shaded areas that are invaded.  You can read more here, and here.

The study emphasizes that when we discuss the effects of invasive species, we should discuss local, regional, and global scale effects, as the outcome for biodiversity is different on these levels.  We should also be aware that invasive species will affect different taxa in different ways.  In the end, the study offers some hope that invaded ecosystems retain the capacity to recover with some management assistance, as most of the native species are still there, holding on.

white tailed deer, fotopedia (http://www.fotopedia.com/items/flickr-1409885673)

White tailed deer is a species native to Kentucky, where I live, and to much of the eastern hardwood forest region of the USA.  However, white tailed deer have attained such high abundance that they cause major ecological problems normally associated with invasive species.  For example, white tailed deer browse tree seedlings, so in areas with high deer populations, growth of new trees may not occur.  Absence of tree regeneration affects other native species such as birds that prefer to nest in younger trees.  Deer may also heavily browse some herb species, like ginseng, leading to their local extinction.  Species like this can be a real threat to the native ecosystem.  A recent article by Michael P. Carey et al. in Frontiers in Ecology and the Environment discusses the case of such “native invaders.”  The authors explain how white tailed deer and other super abundant, high impact native species present challenges for policy, management, and society.

First, Carey et al. outline how native invaders occur.  Non-native species like wild boar and cane toad are introduced by humans from long distances (or escape local captivity), and then have detrimental impacts on local ecological systems.  Native invaders differ in that they are not transported, but their invasive/damaging stage is often sparked in some way by some human activity.  People may cause high reproduction of a native species, such as providing supplemental food or removing a predator (white tailed deer often have no predators) or competitor.  Second, human-modified habitat may provide a niche that a particular native species is good at exploiting (white tailed deer thrive in fragmented forest).  Lastly, some native species are “stocked” (sport fish and game birds), in which large numbers of animals are bred and intentionally released into the wild.  Therefore, because of humans, populations of native species may achieve very large numbers, especially in particular habitats, without actually leaving their native geographic range.

Carey et al. discuss several native invaders in detail, such as rainbow trout, a popular sport fish.  These fish have been stocked in historically fish-less lakes within their native range, and can wreck the native ecosystem and its co-evolved trophic relationships.  Another problem with stocked populations is with stocking individuals of large size (or some other characteristic), making the population genetically and perhaps ecologically different from previous wild populations.  Using examples, Carey et al. explain how native invaders pose similar environmental problems as non-native invasives, but special problems for scientific research, management, public education, and policy.  One research challenge is to determine how abundant the native species once was (a challenge in much of conservation biology), how much population sizes fluctuated, what historical impacts occurred, and whether current impacts are “off the chart.”   The geographic range and population sizes before Europeans entered North America may be difficult to ascertain, as is determining what is “natural.”  However, this information is necessary for managers to know if/when a native should be considered problematic.

One management challenge is that a native invader may be simultaneously an invader in one part of its range, a threatened species in another part of its range, and perhaps even a non-native invader outside its range (e.g. rainbow trout).  Another problem is to control a native invader.  In the case of one native invasive fish, northern pikeminow, fishermen are rewarded for catching this fish (a reward for being an invasivore!), which can help reduce density.  While this does provide recreation and education about the problem, the underlying cause of the invasive habits of this fish (stream and river management practices) are not addressed.  Furthermore, for some native invasives, there is no evidence regarding whether a control program has measurable effects.  A final issue is that some agencies have to balance conservation and natural resource goals.

A final and large challenge is to convince the general public about the potential harm that native species can cause when they become invasive and to demonstrate the need for occasionally lethal control measures.  Public opinion will often be divided, and media attention can simplify the ecological picture.  Still, a broader discussion of what we mean by “invasive,” and a thoughtful reflection on the social and ecological reasons behind the control measures we use (including eating!), can help advance our understanding and management of native invaders.

I recently subscribed to the Ecological Society of America’s podcast “Beyond the Frontier,” which features engaging interviews (usually 10 to 15 minutes, perfect timing for the walk to work) with scientists who have recently published in one of ESA’s journals.  One of my favorite episodes is an interview with Dr. Richard Shine at the University of Sydney, about the cane toad invasion of Australia, one of the most well studied and documented biological invasions.  In the short but insightful interview, Dr. Shine discusses some practical and thoughtful options for controlling cane toads.  Using the toads as a case study, he also discusses public attitudes and actions towards invasive species, and the importance of engaging with community groups.

Cane toad image courtesy of Wikimedia Commons

First Dr. Shine describes the extent of cane toad “hysteria” that has occurred in parts of Australia, due to impacts on native species (especially predators that try to eat them) and the development of general and strong public aversion to the toads.  The public is very aware of the invasion.  Many local groups actively try to eliminate the toads by collecting and euthanizing adult toads (“toad busting”).  Dr. Shine points out that even though the goal (fighting invasive species) is admirable, the methods are ineffective because a single female toad can lay tens of thousands of eggs; collecting single toads, even with hundreds of volunteers, cannot keep up with this rate of reproduction.

Importantly, the interview then turns to discussing the diversity of viewpoints that people have about invasive species- fear, affection, anthropomorphism, naturalistic interest, moralistic, an attitude of dominion.  Different public groups may have different feelings about invasive wildlife (or plants) and thus different approaches to dealing with the issue- some local groups may fight against the invasive species while other groups fight to protect them on grounds of humane treatment or because they prefer the introduced species (e.g. for economic or ornamental value).  This is the case for the coqui frog in Hawaii and the grey squirrel in Europe.  Such viewpoints contrast with those of the researcher, whose goal is to use logic and experimental results to understand ecological processes or impacts.  Shrine emphasizes that to effectively explain their research and how it supports certain control practices over others, the researcher must understand the viewpoint of the different public groups.

Dr. Shine emphasizes that scientists must talk (or blog!) to the public more, as public opinion then trickles up to local and regional public leaders.  Scientists must also understand the pressures on local leaders- economic, social and political.  For example, political leaders are expected to take some immediate action, such as collecting and killing toads, even though the science may say that such removal has no real effect.

Towards the middle of the interview, Dr. Shine then discusses his research into cane toad ecology and the many alternative methods of controlling the invasion- utilizing predators and parasites of the toads, exploiting toad pheromones to attract or stress them, supporting the competitive native toads, and fostering vegetation around the water bodies where they go to reproduce.  These solutions, often using the native species in the fight, can be integrated for a strong, multifaceted, science-based response.  However, such an applied research program is not so common among academics (or funding from science agencies), and thus there is a knowledge vacuum about effective control methods in various situations.  In addition, Shrine notes that control approaches must be explained to the public and political leaders, and actions must be undertaken in dialogue and cooperation with local groups, as also emphasized in a recent article in Conservation Biology.

The discussion concludes by emphasizing that the emotional response of the public is one of the main factors determining whether an invasive species control effort succeeds or fails.  But don’t take my word for it, go listen to the podcast or read the accompanying article in Frontiers in Ecology and the Environment.

Dry riverbed in Australia, courtesy of http://freeaussiestock.com

Climate change and invasive species are often listed as two of the greatest threats to biodiversity. Is it possible that changing climate can itself cause more frequent and damaging invasions, such that the two factors have synergistic effects?  A recent article in Frontiers in Ecology and the Environment by Jeffrey M. Diez et al. approaches this question by examining how extreme climatic events might influence species’ invasions.  This kind of question seems especially important as the United States faces the largest drought in 50 years.

Extreme climatic events (ECEs) are events such as droughts, floods, and storms that throughout historical records are typically quite rare, but may now occur more frequently as global climate changes.  Importantly for biological invasions, these events are not only defined by our own human perspective (e.g., what we call 100 year floods), but they are also defined by the organisms affected by them.  An ECE therefore might be an event that exceeds a species’ physiological limits for heat, salt, etc.

To understand how ECEs might affect invasive species, one must examine each part of the invasion pathway: introduction, establishment, geographic spread, and growth to levels that cause damage.  ECEs such as storms or floods may increase introduction rates by moving more propogules further distances or with more regularity.  This is especially well known in the case of floods that have released species from aquaculture, such as black carp.  Establishment and growth of invasives may be affecting whether ECEs kill native species (that would be their competitors or predators), provide a pulse resource opportunity (such as light in a forest gap), or stress native species (making them less competitive).  ECEs may have indirect effects, such as storms that transport diseases or their vectors long distances, the arrival of which may stress native species that are not directly affected by the ECE itself.

Diez et al. present many examples of ECE-invasion synergisms including: hurricanes which buried native seagrass, allowing for establishment of invasive seagrass; heat waves that led to higher mortality of native than non-native mussel species; drought and salt stress that favor Tamarix over native species; and invasive Bromus grasses that are able to recover quickly from drought, allowing them to invade areas where forests experienced a die-off.

Diez et al. stress that not all ECEs facilitate invasion.  Examples of cases where ECEs actually prevent invasion are also numerous, including native fish that are more favored than invaders in the face of floods and Hawaiian grasses that are more drought tolerant than invasive counterparts.  Then there are more complex interactions.  One interesting example is that even though invasive Taramix perform better than natives in drought, the native Populus species can recolonize faster after extreme floods, so the outcome in this system depends on which ECE occurs more often!  Another interaction is when an invasive species such as cogongrass are more prone to fire, which if combined with dry, hot ECEs, can increase the intensity and frequency of fire, causing cyclical damage to native species.

The article closes by stressing that although there are now numerous case studies of invasive species interacting with ECEs, it is not possible to make general conclusions or predictions.  An important and interesting question is, what makes native ecosystems resilient (able to recover from an ECE)?  It is possible, for example, that communities or species that have evolved under highly variable environmental conditions will be especially resilient.  Another interesting possibility is that invasive species benefit from singular ECEs, but not repeated one.  For example, while invasive plants may respond well to rare drought events, due to fast growth and high leaf area, they will not prosper under sustained or repeated events.

In any case, ecosystem managers must consider that invasion probabilities will change with ECEs.  Some species will be more likely to invade, and some ecosystems will be more susceptible than others.  Fortunately, it is likely that targeted management efforts can increase the resilience of native communities, and, with more research, predict which species are most likely to establish or expand after an ECE.

When it comes to invasives, three key words are prevention, prevention, and prevention.  We know that an efficient and strategic way to combat invasive species is to keep them from establishment in the first place, such as through quarantines, screening mechanisms, or black lists.  A useful question for such prevention policies is, “Which are the probable future invaders, where and how should we target our efforts?”  Well, particular traits (high resource allocation to reproduction, wide environmental tolerance) and a “bad reputation” (invasiveness elsewhere) can be useful predictors.  Now, a recent article in Frontiers in Ecology and the Environment demonstrates a new and interesting way of predicting future invasives.  They analyze trends in the global trade and climate change to identify emerging scenarios of foreign plant introduction and establishment, e.g., where are plants coming from, where are they being planted, and what are their characteristics.

The authors focus on plant introductions to the USA via the horticulture trade- legal sale of ornamental species.  Among these species, climate change is likely to reduce need for cold-hardy plants and increase the need for drought-tolerant plants.  Simultaneously, several regions of the world are rapidly increasing their involvement in global trade, primarily the Middle East, the tropics, and Eastern Europe.  Putting these two facts together, one sees that “global change will influence not just the success of introduced plants but the introduction process itself.”    As most new plant introductions occur soon after trade partnerships are formed, these regions are expected to contribute a “wave” of new introductions.

Desert plants in southern California. From wiki commons

This may be especially relevant for the relatively dry, low population density Central-Western USA, where introduced species currently make up 10 to 15% of the flora, compared to 25 to 30% for many mesic, Eastern states.  Human populations are now expanding in the West, which will likely increase propogule pressure of non-natives.  Gardeners are also planting more drought-tolerant species (called xeriscaping), due to increased recognition of water scarcity. The authors point out that there is much potential for native species to play a role in xeriscaping, but a survey of nursery catalogs shows an uncertain future: despite a trend for greater use of native species, more than half of currently offered drought-tolerant species are non-native.

The authors also look at temperature trends, specifically at a northward shift in hardiness zones, a way of predicting where plants can survive, based on low winter temperatures.  Strong northward shifts are expected for warmest zones 8 and 9.  The authors predict two main consequences for these areas: native species will show decreased fitness in these areas while newly introduced, pre-adapted non-natives can show increased performance.  This imbalance in competitive ability makes invasions more likely here.

Tne main conclusion is that emerging trade partners have climatic conditions just suited to trends in changing temperature and water conditions in the USA, so policy should focus on these regions and types of species.  One policy is weed risk assessments, a pre-emptive examination of an imported species’ ability to invade, to create “white lists” of allowed species.  Unfortunately, information on biology, ecology and invasive potential of species new to the horticulture trade will likely be low.  The authors caution that low information does not mean low invasion probability.  The authors propose increasing communication between invasive species biologists and the buyers and sellers of plants, especially to identify and encourage native alternatives for gardening.  In the other direction, nurseries can share information with biologists about the results of field trials of new plants.  Collaboration among industry, local gardening clubs, government and scientists, could effectively prevent many new invasions.

Effective prevention might spell the end of our invasivore diets, but I think we would agree that would be for the best!

A recent study in the journal Biological Invasions, When are eradication campaigns successful? A test of commonly held assumptions, yielded some invasivore-friendly conclusions. Pluess et al. (2012) analyzed 136 eradication campaigns against 75 different species of invertebrates, plants, and plant pathogens to see what the successful campaigns had in common. The review examined several factors, including how long it took after the invasive organism arrived before the eradication campaign started, the spatial extent of the eradication campaign, how much previous biological knowledge was available on the invader, biogeographic region of the world, and if the invasion was on an island or a continent. The authors were surprised to find that the extent of the eradication campaign was the only significant predictor of a campaign’s success. Campaigns to eradicate an invasive species at the local scale were much more likely to be successful than those at the national scale. The authors suggest this is because local campaigns assess a smaller area and thus perhaps occur before the invader has become established in the ecosystem.

So, act fast. Act local.

Recently, we had the opportunity to sit down with Dr. David Costello to talk earthworms, research, and invasivory. 

Can you start by giving us a quick overview of your dissertation research?

My dissertation research focused on how the impact of invasive species can extend outside the typical boundaries of an ecosystem.  For example, invasive earthworms are typically thought of as a terrestrial problem, but my research showed that the way earthworms change nutrient cycling can cause excess nitrogen to enter adjacent streams.  In general, I found that if you are trying to manage invasive species in an ecosystem, you need to be aware of what is going on in the surrounding areas, even if they are completely different ecosystems.

When did you first become interested in invasive species research?

As an undergraduate at Hobart College, I got my first exposure to invasive species during a summer research internship after my sophomore year.  We conducted a survey of Seneca Lake trying to correlate zebra and quagga mussel densities to lake characteristics.  I spent the following semester studying biology abroad in Australia where the imprint of invasive species, like European rabbits, is really severe.  Both of these experiences sparked my interest in invasive species issues and research in general.

Returning to your dissertation research- have you eaten earthworms?  How did you cook them?  Can you describe your first bite?

I have eaten earthworms a couple times.  In my experience, I’ve had some success blanching them before battering in flour and deep-frying.  It is tough to get earthworms completely clean so I would describe my first bite as “gritty”.  If you can get all the dirt out of them, I think earthworms wouldn’t taste too bad.  For now, I’ll stick to gummy worms!  (Editor’s note: check out some tips on preparing earthworms here)

Dr. Costello samples a deep-fried earthworm

Do you have any other eating invasive species experiences you’d like to share?

I really enjoyed eating rusty crayfish while doing my graduate work at the University of Notre Dame Environmental Research Center in the upper peninsula of Michigan.  My favorite cooking technique involved simply throwing them on the grill and letting them steam in their own shell (technically, an exoskeleton).  It gave them a nice smoky flavor and the meat was still pretty juicy.

What have you been working on since you graduated?

I am currently a postdoctoral research fellow at the Cooperative Institute for Limnology and Ecosystem Research (CILER) at the University of Michigan.  For now I am taking a break from invasive species and working almost exclusively on chemical pollution.  I am working on a number of projects that focus on metal contamination in sediments, which is often the last thing to be cleaned up after a mine or industrial plant closes.  We are exploring the chemical and physical process that cause metal buried just below the surface to be released and how this can potentially affect organisms living on the bottom of aquatic ecosystems.  Invasive earthworms are not far from my mind and I am looking forward to resuming that research.

Any plans to eat any of your study organisms in the future?

I think it is a fun idea as long as your study organism isn’t endangered or poisonous.  I don’t have any specific study organisms right now but for any new projects in the future that will definitely be a consideration!

Would you eat genetically modified invaders?

A recent article from the University of Arkansas demonstrates that genetically modified (GMO) varieties of Canola make up 80% of the escaped feral canola plants identified along roadsides in North Dakota.  The paper by Meredith Schafer and colleagues came out last week in the highly ranked free journal PLoS ONE.   Brassica napus, the plant of the mustard family from which we get Canola oil (and a very close relative of our previously highlighted Field Mustard) is planted on over 30 million hectares worldwide (that’s 115,000 square miles, or about 60 Texases).  Over 90% of the canola crop planted on farms across the United States are genetically modified for pesticide resistance.  And now, so are the “natural” populations.

There’s been a heated discussion over the use of GMO is the nation’s and world’s food supply.  There’s an ongoing campaign for the FDA to require labels on food that contain GMO products (start here and watch the clever video).  Our fellow invasivores at Food and Water Watch  just reported that, yeah, pretty much everything we eat has GMO products, and we don’t know it.   The debate about using GMOs is well-covered elsewhere, and we will stay out of it for now.  But one question about GMOs seems particularly relevant to invasivores…

Do invasivores eat GMOs?

Without GMOs, edible invasive species are pretty much defacto organic.  (Though we always warn about being careful where you harvest certain species (like phragmities) to avoid some harmful contaminants!)  Where do GMOs fit in?  Escaped GMOs are potential invasivore targets, but some people may think twice.  It occurs to us that the population of people who would eat invasive species likely overlaps with those who avoid GMOs.  Where do you stand?

This last week I’ve been in Cairns, Australia, in tropical Northern Queensland.  I’ve been searching for a prime invasivore target: Tilapia.  Tilapia mariae and Oreochromis mossambicus where introduced into Australia several decades ago, and started becoming a problem about 20 years ago.  In many areas, these tilapias have come to dominate native fish communities.  Alas, it remains a crime in pest-aware Australia to possess tilapia, dead or alive.

In the Cattana Wetlands, both T. mariae and O. mossambicus are well established. Catching tilapia is, unfortunately, not allowed.

I’m not here to harvest tilapia, but instead test a new method for detecting them, environmental DNA, also known as “eDNA.”  Using eDNA, it’s possible to detect the presence of invasive species much faster and at much lower densities than is possible using standard methods like nets or electroshocking.  This method has recently been used with much success to detect the spread of Asian Carp up the Mississippi river and approaching the Great Lakes via the Chicago shipping and sanitary canals.

We’re testing this new method to try and better understand the distribution of tilapia in Queensland, and hopefully help target control efforts to prevent this species from establishing in Australia’s grand Murray-Darling Basin and the Gulf of Carpentaria, and evaluate how well previous eradication attempts have succeeded.

I’ve just finished up in Cairns, and now I’m off to Brisbane, another epicenter of the tilapia invasion.

Omega-3 fatty acid content of some edible invasive fish compared to other commonly consumed fish. Data compiled from the NYT, Morris and others 2011, and Karapanagiotidis and others 2006.

On the front page of Monday’s New York Times, a remarkable amount of real-estate was was dedicated to “Another Side of Tilapia, the Perfect Factory Fish.”  This article does a great job highlighting some of the unintended consequences of the massive surge in tilapia production around the world in the last ten years or so.

In the article, one of author Elisabeth Rosenthal’s main points is that farmed tilapia isn’t as chock-a-block full of heart-healthy omega-3 fatty acids as, well, pretty much all other types of fish people consume.  And it got me thinking: What about some of the edible invasive fish we’ve highlighted?

Frankly, the raw omega-3 fatty acid numbers are not encouraging for tilapia and lionfish, but looking great for salmon.  It does not mean that tilapia and lionfish are unsafe, only that they are less than optimal.  There’s other issues though too.  Researchers say the ratio of mega-6:omega-3 fatty acids is a more important measure, which tends to make wild fish more beneficial than farmed fish in general.  So it’s more difficult to compare than either my figure, or Rosenthal’s, suggest.

One of the things we can hope comes from this news is that the enormous demand for tilapia, which drives it’s world wide invasion, may begin to abate.

Morris and others 2011
Karapanagiotidis and others 2006